It is know that many tanks containing volatile products must be periodically purged and that it is then necessary to render them inert for safety reasons. This is in particular the case of the containers or tanks of ships transporting LNG or LPG after the cargo has been delivered.
In the conventional technique, this purging operation, which is often termed "deballasting", is carried out in two stages: bringing the tanks to surrounding temperature at sea and then, alongside the quay, injecting a neutral gas such as nitrogen. During this latter operation, the combustible gas expelled from the tanks under the effect of the thrust of the nitrogen, formed of light hyrocarbons, is sent to the flare. At the end of the deballasting operation, combustible gas is injected moreover into the mixture issuing from the tanks so as to obtain at the flare a stable combustible mixture until the complete stoppage of the deballasting.
This technique, which is at present very widely used, is unsatisfactory since it results, on one hand, in loss of time and money (useless circulation of the ships) and, on the other hand, in losses of fuel, estimated to be between 2% and 5% of the cargo, depending on the liquified gas storage pressure. Similar drawbacks are met with in the other aforementioned cases.